According to known cellular telephone systems, the transition from analog to digital communication within the same frequency band may be performed by replacing seriatim each of the fixed analog channels with three digital channels while maintaining the station sites, cell sizes, and frequency reuse plans which were designed for the analog system having a carrier-to-interference ratio C/I approximately equal to 18 dB. A system having a C/I ratio equal to 18 dB is one in which the signal strength of the carrier is designed to be 18 dB above the signal strength of the interference occurring at the cell borders.
A transition from analog to digital communication in the cellular telephone system according to the method described above implies the following conditions. Initially, the capacity increase of the system is limited to a factor of 3.7 by the unchanged frequency reuse plan. This factor accounts for the value of 3 due to the replacement of one analog channel by three digital channels, and the trunking of three times as many channels in one group. The result being, according to an Erlang table, a total increase of capacity by a factor of 3.7. While being relatively awkward, it is, of course, possible to introduce a new frequency reuse plan at a later time. The operator of the cellular telephone system may desire to obtain equipment from a supplier or manufacturer different from the one who supplied the analog system in place to adapt digital communication capabilities. This is an expensive and cumbersome option in that in order to achieve compatibility between the analog and the digital components, the new supplier must replace the fixed analog equipment in addition to providing new digital equipment.
It is, therefore, desirable to have an alternate method of introducing digital channels where the fixed analog system may be left unmodified, while introducing a completely independently digital system. Furthermore, it is desirable to have the option of utilizing the same or separate station sites, the same or different cell sizes, and the same or different frequency reuse plans.
With respect to cell size, it may be an advantage in an early stage of digitalization of the cellular telephone network to sue digital cells which are larger than the existing analog cells. This configuration will require less equipment while maintaining digital coverage over a large area. Conversely, during a later stage of digitalization of the cellular telephone network it may be an advantage to use analog cells which are larger than the digital cells because at that time the analog cells will be handling less communication traffic than the digital cells.
With respect to frequency reuse, it is possible to plan for a C/I ratio of 10 dB in systems using digital transmission due to the redundancy in the digital code which results in good transmissions occurring at higher disturbance levels than what is possible when using analog transmission. Planning for a system with a C/I ratio of 10 dB enables a denser frequency reuse plan and therefore a higher number of cells per site, which in turn results in higher communication traffic per unit area. The primary purpose of introducing digital voice channels is to increase the communication traffic capacity of the fixed land system, i.e. to allow the land system to serve more mobile stations in a given area. This result may be achieved by combining the aforementioned method of replacing one analog voice channel with three digital voice channels with the method of introducing a denser frequency reuse plan for digital voice channels. In addition, the proposed alternative method of introducing digital voice channels allows the operator to leave the fixed analog system unmodified, except for the canceling of certain assigned frequencies, while new digital equipment is procured which will take over these frequencies.
In existing analog cellular systems, only one set of control channels is generally utilized. The set of control channels comprises dedicated control channels (DCC), paging channels (PC) and access channels (AC). These control channels are used for setting up calls, informing the land system about location and parameters of mobile stations, and for informing the mobile stations about the parameters of the land system. All mobile stations scan and read one of the dedicated control channels when the mobile stations become active or after a certain time has elapsed, or when ordered, or even when the mobile stations enter a new geographical part of the cellular system. The information broadcast by the land system includes a sequence of reserved bits, which are set to an inactive value, e.g. 0, with the intention of enabling future extensions of the protocol.